The invention relates to a novel optical disk and, more particularly, to that disk, to a method of recording and reading information on the disk and to apparatus for carrying out that method.
Optical disks have been used as mass storage devices for computer applications, and such optical disks are known as CD-ROMs. The disk which is used as the CD-ROM is modeled after the standard compact disk (CD) that has been developed for audio applications and is basically an audio CD with various improvements and refinements particularly adapted for computer applications. Using such a CD as a standard, the CD-ROM has a data storage capacity of about 600 Mbytes. By using audio CD technology as its basis, the, CD-ROM and its disk drive have become relatively inexpensive and are quite popular.
However, since conventional audio CDs with their inherent format and storage capacity have been adapted for CD-ROMS, it has heretofore been difficult to improve the data storage capacity. In typical computer applications, a capacity of 600 Mbytes has been found to be insufficient.
Also, the data transfer rate that can be obtained from audio CDs generally is less than 1.4 Mbits/sec (Mbps). However, computer applications generally require a transfer rate far in excess of 1.4 Mbps; but it is difficult to attain a faster transfer rate with conventional CD-ROMS.
Yet another disadvantage associated with conventional CD-ROMs, and which is due to the fact that the audio CD format has been adapted for computer applications, is the relatively long access time associated with accessing a particular location on the disk. Typically, relatively long strings of data are read from audio CDs, whereas computer applications often require accessing an arbitrary location to read a relatively small amount of data therefrom. For example, accessing a particular sector may take too much time for the CD controller to identify which sector is being read by the optical pick-up.
A still further difficulty associated with CD-ROMs, and which also is attributed to the fact that such CD-ROMs are based upon audio CD technology, is the error correcting ability thereof. When audio data is reproduced from an audio CD, errors that cannot be corrected nevertheless can be concealed by using interpolation based upon the high correlation of the audio information that is played back. However, in computer applications, interpolation often cannot be used to conceal errors because of the low correlation of such data. Hence, the data that is recorded on a CD-ROM must be encoded and modulated in a form exhibiting high error correcting ability. Heretofore, data has been recorded on a CD-ROM in a conventional cross interleave Reed-Solomon code (CIRC) plus a so-called block completion error correction code. However, the block completion code generally takes a relatively long amount of time to decode the data, and more importantly, its error correction ability is believed to be insufficient in the event that multiple errors are present in a block. Since two error correction code (ECC) techniques are used for a CD-ROM, whereas only one ECC technique is used for an audio CD (namely, the CIRC technique), a greater amount of non-data information must be recorded on the CD-ROM to effect such error correction, and this non-data information is referred to as "redundant" data. In an attempt to improve the error correction ability of a CD-ROM, the amount of redundancy that must be recorded is substantially increased.